The present invention relates to a chemically modified polyolefin which has good adhesion to metals and to other polar substances.
Polyethylene and polyolefins generally have poor adhesion to metals and to materials containing polar groups. Somewhat better adhesion has been achieved in certain instances by copolymerizing unsaturated esters such as vinyl acetate or alkylacrylate (methylacrylate, ethylacrylate, butylacrylate, etc.), but even these copolymers display insufficient adhesion in most multi-ply structures.
Many ways to improve adhesion have been tried. For example, unsaturated acids or acid anhydrides have been used as comonomers, particularly in the polymerizing of copolymers or terpolymers. Examples of commercial applications include ethylene/acrylic acid copolymer containing 9% acrylic acid, and ethylene/methacrylic acid copolymer containing 9% methacrylic acid. One known commercial terpolymer contains 4% acrylic acid and 7% butylacrylate. A product in which the methacrylic acid is partially neutralized to a salt so that a so-called ionomer has been obtained, is also commercially available. An application in which maleic acid anhydride has been grafted to LDPE, HDPE, or EVA, is an example of the use of maleic acid anhydride for better adhesion. Another example is the terpolymer of ethylene, butylacrylate and maleic acid anhydride.
Direct copolymerizing of ethylene with an unsaturated acid is disadvantageous for a number of reasons. The acids are corrosive and curtail the life span of the processing apparatus, both in the synthesis step and in the working step. Moreover, these ethylene/ acid copolymers or terpolymers are thermally unstable, and low work- ing temperatures must therefore be employed, leading to restrictions on the production rate and product quality. Large acid quantities are indispensable for reaching satisfactory adhesion, because these copolymerized acids are immobile, and it is thus difficult for the acids to force themselves out from the polymer matrix to reach the polar groups present in the other boundary layer.
Large acid quantities are also required in order to lower the crystallinity, this being indispensable if adhesion is to be achieved. This can also naturally be achieved by using a third monomer, such as butylacrylate or vinyl acetate, for example. However, terpolymers are uneconomical regarding recovery of the copolymers in the synthesis. As a result, the content of the comonomer which is active regarding adhesion, is limited. The total content of the comonomer is also limited in view of foodstuff compatibility, under the laws of various countries. High vinyl acetate quantities also introduce corrosion problems. Crystallinity may also be lowered by admixing a thermoplastic elastomer, such as polyisobutylene (PIB), for example. It has been endeavored to improve the mobility of maleic acid anhydride when grafting the same, by, for example, using spacer groups between the maleic acid anhydride and polymer (Diels-Alder reaction between diene and maleic acid anhydride, and grafting thereof to the polymer).